Exhaust combustion gas generated as a result of combustion, for example, in a furnace of a boiler or the like has its heat recovered by a heat exchanger and fuel economizer while passing through a main duct. Then, the exhaust combustion gas is supplied with ammonia gas for reduction and removal of nitrogen oxides in the exhaust gas and led into a denitrification reactor filled with a catalyst for nitrogen oxide removal.
Here, if sulfur oxides (especially SO3) exist in the exhaust gas and exhaust gas temperature is low (normally below somewhere between 250 and 330° C. although this depends on concentrations of ammonia and sulfur oxides in the exhaust gas), the sulfur oxides react with ammonia gas, resulting in deposition of acidic ammonium sulfate.
The acidic ammonium sulfate, which is in a liquid state, adheres to surfaces of the catalyst and thereby covers active catalyst sites, resulting in performance degradation of the catalyst. Thus, operation of the exhaust gas denitrizer is conditional on keeping the temperature of the exhaust gas flowing into the exhaust gas denitrizer above the temperature at which acidic ammonium sulfate starts to deposit.
To satisfy this condition, it is a known practice to install a bypass duct which branches off from a main duct on an upstream side of a fuel economizer and rejoins the main duct on a downstream side of the fuel economizer as described, for example, in Patent Literature 1. That is, bypass exhaust gas which bypasses the fuel economizer by flowing through the bypass duct is maintained at high temperature without being cooled, and consequently, the temperature of the exhaust gas after rejoining main exhaust gas flowing through the main duct can be kept above the temperature at which acidic ammonium sulfate starts to deposit.